1. Field of the Invention
This invention relates to a system and method for detecting crystallographic structures in metals. More specifically, the invention relates to a system and method for detecting surface and subsurface crystallographic grain defects in directionally solidified and single crystal castings used in gas turbines and other high temperature devices.
2. Description of the Related Art
It is known in the field of casting metal parts that directionally solidified (DS) and single crystal (SC) castings provide enhanced strength over traditional polycrystalline castings. This is primarily due to the beneficial directional grain structures and the absence of transverse grain boundaries in the castings. In the field of gas turbines, SC castings of blades and vanes have enhanced resistance to creep and low cycle fatigue compared to DS and polycrystalline castings. This is primarily due to the elimination of grain boundaries and more strengthening elements added to the SC metals.
Due to the critical nature of SC blades and vanes, these parts are inspected closely for grain defects. Some defects can cause reduced life of the part and catastrophic failure. The grain defects can be freckles, secondary grains, recrystallized grains, low and high angle boundaries, slivers, and zebras. Freckle defects are chains of equiaxed and randomly oriented grains. Secondary grains are redundant and crystallographically misoriented grains. Recrystallized grains are grains that have formed during high temperature heat treatment due to prior cold working on the part or residual stresses from casting. Low and high angle boundaries are surfaces between two misoriented grains which appear as lines of different reflective contrast. Sliver grains are narrow, elongated grains with low misorientation. Zebra grains are multiple, thin grains on platforms with low angle boundaries.
The negative effects of defects are augmented in SC parts compared to DS parts as a result of the alloying compositions of the parts. In DS alloys, elements such as zirconium, boron, and carbon are used to enhance the strength at grain boundaries, which enhances the strength of the parts. In SC alloys, zirconium, boron, and carbon are not used so that optimized heat treatment techniques can be used for enhanced part strength. However, this results in weaknesses in a SC part at the grain boundaries when grain defects do occur.
Directionally solidified parts are also sensitive to the presence of transverse grain boundaries. Grain defects, such as freckles and recrystallized grains, can significantly reduce the parts' strength.
The grain defects can be surface defects, subsurface defects, or both. It is believed that a vast majority of defects extend to the surface of a part, especially in smaller, thinner parts. Currently, surface defects can be detected by visual inspection after a surface acid etching of the part. Laue back scatter x-ray diffraction analysis has been used to determine the mismatch of crystallographic orientations and the severity of the defects and whether the part is suitable for use. The prior art discloses using an x-ray film to analyze the back scattered x-ray diffraction patterns. However, both the acid etching and using an x-ray film for back scatter diffraction analysis is time consuming and limited to surface defects, restricting its use as a tool for 100% inspection of parts.
It is also believed that some grain defects are entirely subsurface, especially for larger SC and DS parts. However, other than destructive analysis of the part, no method exists to determine the presence, location, size and degree of severity of these subsurface defects. It is possible that parts with major defects pass nondestructive inspections, including surface inspections, and are employed in turbines and other devices, which may result in part failure. Therefore, it is desirable to provide a quick system and method for detecting subsurface crystallographic grain structures and defects in a sample.